This invention relates to a device for the measurement of ionizing radiation dose. More particularly, the present invention relates to a device for measuring high-level ionizing radiation dose, in which the junction field effect transistor is adopted as a dose detector to effect the measurement of the absorbed radiation dose particularly in the high-level range of from 10.sup.6 to 10.sup.9 rads by utilizing the radiation effect on the junction field effect transistor (hereinafter referred to briefly as J-FET).
The devices which have heretofore been developed for the measurement of high-level ionizing radiation dose include a chemical dosimeter which makes use of the chemical change caused in a sample in the form of a solution by the ionizing radiation, a thermal fluorescent dosimeter which utilizes the phenomenon that the energy entrapped by radiation within the measuring element is emitted as fluorescent light upon a thermal treatment, a plastic film dosimeter which utilizes the discoloration of plastics, such as polymethyl methacrylate and blue cellophane, caused by the ionizing radiation, glass dosimeter which utilizes the formation of color centers in glass, a solar battery dosimeter which utilizes the change in resistance of a solar battery and a hydrocarbon dosimeter which utilizes the chemical change in hydrocarbon. For the purpose of measurement of high-level ionizing radiation dose, all these conventional devices combine merits and demerits.
To be specific, most chemical dosimeters are capable of measuring radiation dose to the level of about 10.sup.7 rads at most and some of the hydrocarbon dosimeters permit dose measurement up to 10.sup.9 rads. With these dosimeters, the test specimens cannot be reused, the treatments involved consume much time and the specimens required in the detection units cannot easily be reduced in size. With thermal fluorescent dosimeters, measurement of radiation dose beyond the level of 10.sup.6 rads is not possible. With plastic dosimeters and glass dosimeters, the proportional relationship between the dose of radiation and the response is lost beyond the level of 10.sup.7 rads. These dosimeters cannot be used for the measurement of dose higher than 10.sup.8 rads and do not permit re-use of test specimens. With dosimeters utilizing solar batteries, although the proportional relationship between the ionizing radiation dose and the response is obtained over a wide range of from 10.sup.4 rads to 10.sup.8 rads, the dosimeters themselves have a disadvantage of being usable only within narrow temperature and humidity ranges and do not permit easy dimensional reduction.
The J-FET is extensively used as the principal element in low-noise low-frequency amplifiers. It has been ascertained in the art that the noise of the J-FET is closely related to the defect centers causable such as by inclusion of impurity in semiconductor. The inventors have made a discovery that the defect center generated in the depletion layer near the channel layer in consequence of the irradiation of the ionizing radiation to J-FET functions to enhance the noise in the same way as the defect center of the type causable such as by inclusion of impurity and that the change in the square of the noise voltage is directly in proportion to the amount of the irradiated radiation dose or to the irradiated radiation fluence. This discovery has led to the present invention.
An object of the present invention, therefore, is to provide a device which permits easy measurement of high-level ionizing radiation dose of the order of from 10.sup.6 to 10.sup.9 rads.
Another object of this invention is to provide a device which uses a highly compact measuring unit and yet permits easy measurement of high-level ionizing radiation dose.
A further object of this invention is to provide a device for the measurement of high-level radiation dose which permits ready regeneration of the measuring unit.